Phase-only filters and "phase dominant" spatial light modulators (SLMs) are important new devices for optical signal processing applications. See for example U.S. Pat. No. 4,826,285 issued to Joseph Horner and his paper published in "Applied Optics", Vol. 24, No. 5, pp. 609-611. Techniques are needed for phase-dominant SLMs to enhance the relatively small phase shifts that are found in active semiconductor material. Although there is a large body of literature on FP elements as enhancers of amplitude modulation (AM), scant coverage has been given to the phase modulation (PM) properties of Fabry Perot (FP) resonators, an oversight we hope to remedy.